Up-regulation of type XIX collagen in rhabdomyosarcoma cells accompanies myogenic differentiation

Rhabdomyosarcomas are known to recapitulate some of the early events in skeletal muscle embryogenesis, and cultures derived from these tumors have been extensively used to elucidate processes associated with the differentiation of primitive mesenchymal cells. These neoplasms have also provided important systems for studying different collagen types. This aspect is particularly relevant to type XIX collagen, which was originally identified from rhabdomyosarcoma cDNA clones. Although this collagen has been localized in vivo to basement membrane zones in a wide variety of tissues, including skeletal muscle, the tumor cells appear to be a unique source of its expression in vitro. We have found that one particular cell line-derived from a peritesticular embryonal rhabdomyosarcoma-produced relatively large amounts of type XIX collagen, especially in those rare instances in which these cells appear to spontaneously differentiate. To characterize this phenomenon, tumor cells were grown under conditions known to induce differentiation in normal myoblast cultures. In response to this treatment, the typical tumor cell morphology consistently and reproducibly switched from polygonal to round/spindle-shaped with the subsequent appearance of some structures resembling myotubes. Concurrently, the cultures commenced a dramatic up-regulation of type XIX collagen and skeletal muscle myosin heavy chain and alpha-actinin in a time-dependent fashion, whereas protein and mRNA levels of other matrix proteins were either decreased or unchanged. Moreover, immunocytochemical analysis revealed that only a subpopulation of the cells was responsible for the increased synthesis of type XIX collagen, alpha-actinin, and myosin, and that the same cells which stained positive for the collagen also stained positive for the muscle proteins. Taken together, the results suggested that type XIX collagen may be involved in the initial stages of skeletal muscle cell differentiation.